This invention relates to preparing pre-coated, aluminum alloy-based components. In particular, the present invention relates to the use of fluidized-bed coating processes to coat aluminum-alloy aircraft structural components.
It has recently been discovered that the corrosion protection and ease of installation and processing of certain aluminum-alloy aircraft components can be improved by pretreating the alloys with an organic coating prior to installation. Such pretreatment reduces processing time and commensurate cost, while obviating conventional use of toxic solvents that pose difficult and expensive disposal problems.
Such advances are the subject of commonly owned U.S. Pat. No. 5,614,037 the entire contents of which are incorporated by reference herein. As disclosed therein, it has been the practice to coat some types of fasteners with organic coatings to protect the base metal of the fasteners against corrosion damage. In the usual approach, the fastener is first fabricated and then heat-treated to its required strength. After heat-treatment, the fastener is etched with a caustic soda bath or otherwise cleaned to remove the scale produced in the heat-treatment. Optionally, the fastener is alodined or preferably anodized. The coating material, dissolved in a volatile carrier liquid, is applied to the fastener by spraying, dipping, or the like. The carrier liquid is then evaporated. The pre-coated fastener is heated to an elevated temperature for a period of time to heat treat the alloy, and simultaneously cure the coating. The finished fastener is then used in the fabrication of the aircraft structure.
As explained in U.S. Pat. No. 5,614,037, it has not been the practice to coat high-strength, aluminum-alloy fasteners and other structural components with curable coatings, because it is observed that the normally accepted curing treatment for the coating can adversely affect the strength of the components. The uncoated aluminum-alloy components are therefore more susceptible to corrosion than would otherwise be the case.
The absence of the coating means that aluminum components such as rivets, fasteners, etc., must be installed using a wet-sealant compound for purposes of corrosion protection and ease of installation. The wet-sealant compound typically contains toxic components and therefore requires precautions for the protection of the personnel using it and for environmental protection. It is also messy and difficult to work with, and requires extensive cleanup of the area around the fastener using caustic chemical solutions.
The procedural advances described in U.S. Pat. No. 5,614,037 address many of these needs. However, for certain aircraft components, including fasteners, coating uniformity is essential. While coatings can be applied in many ways, many known application techniques including rack and barrel systems, conveyer systems or plasma vapor deposition do not provide the required tolerances and uniformity for irregularly-shaped aluminum alloy aircraft components such as rivets and other fasteners. For example, if coatings are deposited too thickly to aluminum alloys, the coating's resilience (i.e., the strength or integrity of the coating) is weakened. If too thin a coating is deposited, corrosion protection and overall component performance may suffer. Further, coating application methods where one layer of coating is delivered over many separate applications is unacceptable since the multiple bond layers introduced between coating applications cannot be reliably cured to produce the proper cross-linking necessary to achieve the optimum strength levels required.
Such coating tolerances were previously difficult to obtain using known methods for small, irregularly shaped aluminum-alloy components, such as rivets and other fasteners.
Therefore, there exists a need for an improved approach to the application of corrosion inhibiting coatings to aluminum-alloy aircraft structural components, such as rivets, fasteners, small and irregularly-shaped components, etc., and the mechanical, aircraft structures attached to these aforementioned components.